Topographic and seismic constrains on the post-seafloor spreading magmatism in the Xisha uplift region, northwestern South China Sea

Submarine intraplate magmatism has significant feedback influence on the evolution of sedimentary basins, and may in turn be heavily influenced by pre-existing faults. Furthermore, magmatic products reflect mantle magmatism and processes. Using multi-beam bathymetric data and seismic profiles, this study, describes the morphology and internal architecture of post-seafloor spreading magmatic structures especially volcanoes for the first time in extensive detail in the Xisha uplift of northwestern South China Sea. Approximately 201 volcanoes have been identified in the Xisha uplift. Their average diameters range from 0.625 to 10.75 km and heights with 0.04 to 1.02 km; the average flank dips range from 7.27° to 31.08°, and the volcanic volume ranges from 0.005 to 19.52 km³. Numerous solitary volcanoes and volcano groups produce distinct seismic reflections and local unconformities with extents of 4 - 35 km that we have mapped (horizons T1 - T2) together with plutons. Horizon T2 (5.5 Ma) shows an increase in amplitude relative to that of Horizon T1 (2.6 Ma) and displays more extensive local unconformities that those of Horizon T1, indicating that the magmatic activity during 5.5 Ma may be more intense than that in 2.6 Ma.

We propose that the post-seafloor spreading magmatism may originate from the Hainan plume, which has been confirmed by tomographic and geochemical data. The rifting of the SCS and the strike-slipping movement of Ailao Shan-Red River Strike-slip Zone and its seaward continuation East Vietnam Boundary Fault may result in possible fracture zones or pre-existing faults in the Xisha uplift, which provide the primary conduits for the upwelling mantle. The intense magmatism was induced by the reverse of strike-slip movement of Ailao Shan-Red River Strike-slip Zone and its seaward continuation East Vietnam Boundary Fault during 5 Ma and finally emplaced by these magmatic activities.